北天山四棵树花岗岩体锆石U-Pb年代学及地质意义
LA-ICP-MS Zircon U-Pb Chronology and Geological Implications of Sikeshu Stitching Pluton in the North Tianshan Suture Zone, Xinjiang
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摘要: 四棵树岩体位于北天山缝合带中，是侵入依连哈比尔尕岛弧带中最大的一个花岗岩体，具有“钉合岩体”的构造意义。组成该岩体的岩石类型较多，大体上可分为3个侵入期次，依次为石英闪长岩和花岗闪长岩、二长花岗岩和钾长花岗岩、花岗斑岩；其中第二侵入期次构成了该岩体的主体。岩石SiO2含量为57.85%～74.28%，K2O含量为1.95%～5.25%，Na2O含量为3.00%～4.50%，整体表现为高K、Ca碱性特征。∑REE=113.91×10-6～169.47×10-6，（La/Sm）N 为1.77～3.16，（Gd/Yb）N为1.33～2.12，（La/Yb）N为3.41～5.64，δEu为0.37～0.82，表现为轻稀土元素富集的右倾型特征；岩石具有Nb、Ta、Eu的亏损以及Th、Zr、Hf相对富集。通过形成环境判别，第一期石英闪长岩和花岗闪长岩大部投在火山弧环境，板块碰撞前—后碰撞隆起阶段；第二期二长花岗岩和钾长花岗岩大部投在后碰撞阶段；第三期花岗斑岩主要投在晚造山阶段。通过LA-ICP-MS对该岩体中石英闪长岩、二长花岗岩和钾长花岗岩进行较为系统的锆石U-Pb测年，得出其加权平均年龄分别为（324.1±4.3）Ma、（314.9±4.1）Ma、（311.5±3.9）Ma。结合前人得出的巴音沟蛇绿岩中辉长岩的（344±3.4）Ma年龄，可初步得出北天山缝合带的形成时代应该为344～311.5 Ma，碰撞造山作用主要发生在324～311 Ma，随后进入后碰撞伸展阶段，而四棵树岩体3期侵入岩可能分别代表了北天山造山带俯冲汇聚-碰撞-陆内造山各阶段构造-岩浆作用的产物。
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关键词:
- 高K、Ca碱性花岗岩 /
- 后碰撞 /
- 钉合岩体 /
- 四棵树 /
- 北天山缝合带
Abstract: Sikeshu granitic pluton is located in the North Tianshan suture zone, the biggest granitic body which intrudes the Yilianhabierga island arc belt, and has tectonic significance as one stitching pluton. Sikeshu pluton is composed of various types of rocks generally formed in three intrusive periods. In the first period, the pluton rocks are quartz diorite and granodiorite, while in the second period the major compositions are monzogranite and K-feldspar granite, and in the third period the pluton is made of granite porphyry. Noticeably, rocks occurred in the second intrusive period constitute the main body of the pluton. On the whole, the rocks are featured by high-K calc-alkaline characteristics, with SiO2 content varying between 57.85 and 74.28 wt.%, K2O content from 1.95 to 5.25 wt.%, and Na2O content from 3.00 to 4.50 wt.%. The total REE content is between 113.91×10-6 ppm and 169.47×10-6 ppm. In addition, the values of (La/Sm)N, (Gd/Yb)N and (La/Yb)N fluctuate from 1.77 to 3.16, 1.33 to 2.12, and 3.41 to 5.64 respectively, showing a right-inclined distribution pattern characterized by weakly negative Eu anomalies with δEu value from 0.37 to 0.82. Moreover, the rocks are enriched in Th, Zr, Hf and depleted in Nb, Ta and Eu. In terms of deformation environment, quartz diorite and granodiorite of the first period mainly fall into the volcanic arc area of the pre-collision to the post-collision stage; rocks of the second period, namely monzogranite and K-feldspar granite, mainly formed in the post-collision stage; and graniteporphyry in the last period occurred in later orogenic stage. LA-ICP-MS zircon U-Pb dating results of quartz diorite, monzogranite and K-feldspar granite are 324.1±4.3 Ma, 314.9±4.1 Ma and 311.5±3.9 Ma. Considering the age of Bayingou ophiolite, which is 344±3.4 Ma according to previous researches, we can conclude that the suture zone of North Tianshan formed between 344 to 311.5 Ma and the major collision orogenic process took place from 324 to 311 Ma, and then the suture moved into the post-collision stage after 311 Ma. To some extent, the three intrusive rock units of Sikeshu granitic pluton may be representative products of three tectonic-magmatic stages, which are subduction, collision and intracontinental orogenic of North Tianshan orogenic belt.-
Key words:
- High-K calc-alkaline granite /
- post-collision /
- stitching pluton /
- Sikeshu /
- North Tianshan suture zone
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